1. Field of the Invention
The present invention relates to an image-reading apparatus for reading a character or pattern image on a document, and more particularly to a line sensor apparatus which supplies image data to an image processor connected to a computer or the like, or which is incorporated in a facsimile machine.
2. Description of the Related Art
In general, an image-reading apparatus for reading a character or pattern image comprises: a document table on which a document having a character or pattern is mounted; and a carriage device movable in parallel to the document table and elongated in the main scanning direction, i.e., a first direction, which can read the image portion within a given document region linearly extending along the main scanning direction on the document. The carriage device contains the following structural components: an illumination device for illuminating a document; a focusing device for focusing the light beam reflected by the document on a predetermined position; and a conversion device, which includes a CCD line reading sensor or a photoelectric conversion element, for converting the reflected light beam focused by the focusing device into an electric signal. These structural components are integrally assembled as one unit. The image-reading apparatus is provided with a driving device for moving either the document table or the carriage device in an auxiliary scanning direction perpendicular to the main scanning direction, so as to successively read the image portions o the document through the document region.
The image-reading apparatus reads an image as follows. First of all, a document is placed on the document table, with the image-shown side facing the carriage device. Then, the document is illuminated with the light-rays emitted from the illumination device of the carriage device. The light beam reflected by the document, i.e., information representing the character or pattern image shown in the document, is focused on the CCD line reading sensor by the focusing device of the carriage device. Thus, the reflected light beam is converted into a electric signal. In this manner, the image portion within the given document region is read in the main scanning direction. Next, the carriage device is shifted in parallel to the fixed document table; alternatively, the document table is shifted in parallel to the fixed carriage device. In this condition, the image portion within the document region is read in a similar manner to that mentioned above. With the above operations repeated, all character or pattern image portions shown on the document are sequentially read from the rectangular region. The electric signals produced by the reading sensor are supplied to a memory or an image processor as image information signals, so as to enable the image to be reproduced or processed.
U.S. Pat. No. 4,670,795 discloses a color image forming apparatus employing a plurality of photoelectric converters or CCD sensor elements. These converters are arranged in two or more rows, and a color-separation filter is provided for each converter. The color image on a document is partly read by one row of the converters arranged in one row, and is then partly read by the another row of the converter.
In the CCD line sensor, a dark current peculiar to the sensor is produced and outputted as part of an output signal even when the sensor does not read image information, i.e., when no external light-rays is incident on the sensor.
The intensity of the dark current varies with time, due to the heat which is produced from a component such as a light source and applied to the sensor or due to the heat which the sensor generates in accordance with a driving current supplied thereto. Since the sensor is elongated in the main scanning direction, the distribution of heat in the longitudinal direction of the sensor may not be uniform; it is locally different in accordance with the structure in which the sensor is fixed. Therefore, the magnitude of an output electric signal obtained in accordance with the image information of the document varies in accordance with the variation in the dark current due to the distribution of heat in the longitudinal direction of the sensor. It should be noted that the electrical correction circuit for correcting the output signal depending on the heat distribution and variation can not easily be designed.
The above U.S. Pat. No. 4,670,795 discloses the use of a plurality of converters, but does not mention any problems concerning heat radiation and how such converters should be operated in a stable condition.